1. Field of the Invention
The present invention relates generally to a transmission controlling method in a mobile communication system, and in particular, to a method of controlling reverse transmission.
2. Description of the Related Art
With the phenomenal growth of mobile communication technology, many mobile communication systems have been proposed and are currently being tried in the field. These systems generally operate based on CDMA (Code Division Multiple Access), and a 1×EV-DO (1×Evolution-Data Only) system called HDR (High Data Rate) is designed to carry out high-rate dedicated data transmission.
Similarly to other systems, 1×EV-DO systems also require appropriate scheduling to efficiently transmit packet data on the forward and reverse links. “The forward link” is a link directed from a base station to an access terminal (AT) or mobile station, and “a reverse link” is the opposite link directed from an AT to a base station. For forward data transmission, the base station transmits data to a particular AT attempting to utilize the best channel conditions available, considering the air link quality status between the base station and 1×EV-DO ATs, and other environments, resulting in a maximized data transmission throughput for the AT. Concerning reverse data transmission, a plurality of ATs access the base station simultaneously. In this situation, the base station must control overload within the reverse link channel capacity by controlling congestion and data flows from the ATs.
Besides the 1×EV-DO systems, other mobile communication systems designed to support multimedia service must also manage reverse data transmission efficiently. In doing so, system performance and capacity must be ensured.
In the existing 1×EV-DO systems, an AT carries out reverse data transmission based on a RAB (Reverse Activity Bit) and a ReverseRateLimit (RRL) message received from a base station, and reports to the base station its variable data rate via an RRI (Reverse Rate Indicator). The RRI indicates to the base station at what data rate the reverse traffic data is being sent. The base station transmits the time-multiplexed channels to the AT: a forward MAC (Medium Access Control) channel, a pilot channel, an FAB (Forward Activity Bit) channel and a RAB channel. The RAB represents the degree of congestion of the reverse link, and a data rate available to the AT varies according to the RAB. The base station controls a data flow from the AT by commanding an increase/decrease in the reverse data rate using the RAB to thereby control overload and capacity of the reverse link. Since the RAB is broadcast to a plurality of ATs, the ATs receiving the RAB double their data rates or reduce them by half uniformly according to the RAB. The transmission time (or transmission period) of the RAB is determined byT mod RABlength  (1)where T is system time and RABlength is the length of the RAB expressed in the number of slots. Table 1 below lists binary values representing RAB lengths. The base station transmits one of the binary values to the ATs and then the ATs calculate a slot time. The ATs receive the RAB on a forward MAC channel (F-MAC channel) using the received RABlength information and the system time.
TABLE 1BinaryLength (slots)00 8011610321164
With the RAB transmitted from the base station to the ATs at the time calculated by equation (1), the ATs determine whether to increase or decrease their data rates for the current reverse transmission.
Despite a data rate increase command from the RAB, the highest data rates the ATs may transmit at may be limited by an RRL message received from the base station. The data rate can also be limited by the transmission power of the ATs. As a result, the ATs do not increase their data rates, wasting radio resources. Although an AT requests a much higher data rate to transmit an increased amount of data, its data rate is increased by a mere one unit because the RAB supports a gradual data rate increase/decrease. Accordingly, the base station must know the status of the ATs for efficient use of radio resources. This implies that the ATs should report their status to the base station. Unfortunately, neither the existing 1×EV-DO systems nor currently proposed mobile communication systems do not provide such functionality.
FIG. 1 is a flowchart illustrating a reverse data rate controlling procedure for an AT in an existing 1×EV-DO system.
The AT sets its lowest available data rate at an initial reverse data transmission. If the current data rate is less than a data rate provided in an RRL message received from a base station, the AT transmits data at the provided data rate after 32 slots (53.33 ms). On the other hand, if the current data rate is greater than the provided data rate, the AT transmits data at the provided data rate. For the subsequent reverse transmission, the AT determines its data rate by the procedure of FIG. 1. The RRL message is transmitted to the AT to determine an initial reverse data rate and reset the reverse data rate.
After determining its data rate, the AT reports its data rate to the base station by an RRI symbol as shown in Table 2. The reverse data rate is selected among 4.8, 9.6, 19.2, 38.4, 76.8 and 153.6 kbps. This reverse data rate is reset by a message such as an RRL message or an RAB message received from the base station. Table 2 below lists RRI mappings in the 1×EV-DO system.
TABLE 2Data rate (kbps)RRI symbol4.80019.601019.201138.410076.8101153.6110
To aid the AT in resetting its data rate, the base station must transmit to the AT an RRL message of the structure shown in Table 3.
TABLE 3FieldLength (bits)Message ID829 occurrences of the following two fieldsRateLimitIncluded1RateLimit0 or 4ReservedVariable
Upon receipt of the RRL message, the AT resets the reverse data rate by comparing the current reverse data rate with a data rate set in the RRL message. 29 records may be inserted in the above RRL message and each record indicates a data rate assigned to a corresponding MACindex among MACindexes 3 to 31. In Table 3, Message ID indicates the ID of the RRL message. RateLimitIncluded is a field indicating whether RateLimit is included in the RRL message. If RateLimit is included, RateLimitIncluded is set to 1 and otherwise, it is set to 0. RateLimit indicates a data rate assigned to a corresponding AT. The base station assigns data rates shown in Table 4 to ATs using four bits.
TABLE 40 × 04.8kbps0 × 19.6kbps0 × 219.2kbps0 × 338.4kbps0 × 476.8kbps0 × 0153.6kbpsAll other values are invalid
During reverse data transmission, the AT monitors a F-MAC (Forward Medium Access Control) channel from the base station, especially the RAB on the F-MAC channel and adjusts its current data rate by performing a persistence test.
Referring to FIG. 1, the AT monitors the RAB of a F-MAC channel from a base station included in the active set of the AT in step 100 and determines whether the RAB is equal to 1 in step 102. If the AT has six sectors/base stations in its active set, the AT determines whether at least one of the RABs of the F-MAC channels received from the six sectors/base stations is equal to 1. If at least one RAB is equal to 1, the AT proceeds to step 112, otherwise, the procedure goes to step 104.
The case that all RABs=0 will first be considered.
If the RAB is 0, the AT performs a persistence test in step 104. The persistence test is available when the base station broadcasts the RAB to a plurality of ATs to control the amount of reverse data from the ATs. The persistence test passes or fails depending on whether a generated random number satisfies a desired condition.
If the persistence test passes in step 104, the AT increases its data rate (TX rate) in step 106. On the contrary, if the persistence test fails, the AT jumps to step 120. The AT increases the TX rate in step 106 and compares the increased TX rate with a maximum allowed data rate (a max TX rate) in step 108. If the increased TX rate is greater than the max TX rate, the AT sets the TX rate to the max TX rate in step 110 and goes to step 120.
Now, the case that at least one RAB=1 will be considered. If the RAB is equal to 1 in step 102, the AT performs a persistence test in step 112. If the persistence test fails, the AT jumps to step 120. If the persistence test passes, the AT decreases the TX rate in step 114 and compares the decreased TX rate with a minimum data rate (a min TX rate) in step 116. If the decreased TX rate is less than the min TX rate, the AT goes to step 118, otherwise, it jumps to step 120. The AT sets the TX rate to the min TX rate in step 118 and goes to step 120. The min TX rate can be a default data rate of 9.6 kbps or a data rate designated at a call connection.
In step 120, the AT generates an RRI symbol corresponding to the set TX rate. The AT transmits the AT along with traffic data only if a traffic connection is opened between the base station and the AT. If the traffic connection is not opened, the AT transmits only the RRI symbol.
FIG. 2 is a diagram illustrating data transmission/reception between an AT and an HDR sector included in the active set of the AT. As seen from FIG. 2, F- and R-traffic channels and F- and R-MAC channels have been established between the AT and sector 1 with a connection opened between them. F denotes forward direction and r denotes reverse direction. No F-traffic channels are assigned to the AT from sector 2 through n with no connection opened between them. In the 1×EV-DO system, the AT can maintain up to six sectors/base stations in its active set. Therefore, the AT monitors F-MAC channels from all the sectors of the active set, especially RABs on the F-MAC channels to determine its TX rate.
Upon receipt of at least one RAB set to 1, the AT performs a persistence test to decrease its TX rate. In the persistence test, the AT generates a random number and compares it with a persistence vector defined by the base station at or during a connection. If the random number satisfies a desired condition, the AT determines that the persistence test passes. The AT then decreases the TX rate by half. On the contrary, if the persistence test fails, the AT maintains the TX rate. If the TX rate is less than a min TX rate, the AT sets the TX rate at the min TX rate. Meanwhile, if all the RABs are equal to 0 and a persistence test passes, the TX rate is doubled. If the persistence test fails, the AT maintains the TX rate. If the TX rate becomes greater than a max TX rate, the AT sets the TX rate to the max TX rate. Also, in the case where the AT is limited in transmission power, it maintains the TX rate. The RAB that functions to double a reverse data rate or reduces it by half is broadcast to ATs in TDM with an FAB on a forward common channel, a F-MAC channel. The ATs increase/decrease their data rates uniformly according to the RAB.
From the system's perspective, the above-described reverse transmission controlling method for the current 1×EV-DO systems simplifies bandwidth control and overhead control. However, the uniform control without considering the individual status of ATs brings about a bandwidth waste and decreases the data transmission efficiency of the ATs. Accordingly, the base station should consider the status of the ATs in controlling their data rates to save bandwidth and provide transmission efficiency. The currently proposed mobile communication systems as well as the 1×EV-DO system all exhibit these problems.